This application is based on and incorporates herein by reference Japanese Patent Application No. 2001-206684 filed on Jul. 6, 2001.
The present invention relates to an electronic device in which a plurality of microchips are stacked.
As an electronic device of this type, an acceleration sensor (accelerometer), which includes a processing microchip, a sensing microchip, and a housing, is proposed. The acceleration sensor is used under such thermally harsh condition as in a vehicle, in which the temperature of the sensor in use ranges between xe2x88x9240xc2x0 C. and +120xc2x0 C.
The processing microchip, which is located in the housing, has a bonding area. On the bonding area, the sensing microchip is located. The sensing microchip includes a substrate made of semiconductor such as silicon and a sensing region, which is located in the substrate. The sensing region is for sensing an acceleration of a target object, which is a velocity change rate of the target object. When the velocity of the target object increases, the acceleration is positive, and when the velocity decreases, the acceleration is negative. Positive and negative accelerations can be detected in the same manner, so hereinafter explanation will be made only for the case that positive acceleration is detected.
In the proposed acceleration sensor, the processing microchip and the sensing microchip are bonded together by a polyimide-type adhesive film. However, as shown in FIG. 5, the elastic modulus of the polyimide-type adhesive film changes relatively significantly at approximately 110xc2x0 C. due to the glass transition of the polyimide-type adhesive film. Therefore, the sensing precision of the sensing microchip is undesirably lowered by the relatively significant change in the elastic modulus of the polyimide-type adhesive film.
The proposed acceleration sensor can cover various acceleration ranges (dynamic range) by combining the sensitivity (output signal per unit acceleration) of the sensing microchip and an amplification factor of the processing microchip. To be specific, the proposed acceleration sensor can cover various acceleration ranges from a minimum range of 0 g""s-0.3 g""s to a maximum range of 0 g""s-250 g""s (the unit g is the acceleration of gravity). However, in the case that the proposed acceleration sensor is designed to cover a relatively low acceleration range, for example, in the case that the upper detection limit of acceleration is in the range between 0.3 g""s and 1.5 g""s, the sensing precision of the sensing microchip is seriously lowered by the change in the elastic modulus of the polyimide-type adhesive film.
In the case that the proposed acceleration sensor outputs a maximum of 5V in the dynamic range covered by the proposed acceleration sensor and the upper detection limit is 50 g""s and the deviation in output signal from the proposed acceleration sensor is Dvol (V), the output signal (V) per unit acceleration (g) is 0.1+0.02Dvol (V/g). In the case that the upper detection limit is 1.5 g""s, the output signal (V) per unit acceleration (g) is 3.3+0.67Dvol (V/g). The sensing precision is seriously lowered by the change in the elastic modulus of the polyimide-type adhesive film when the proposed acceleration sensor has such relatively high sensitivity as in the later case.
The present invention has been made in view of the above aspects with an object to provide an electronic device, which includes two microchips and a bonding layer with which the microchips are bonded together, and in which the influence of the elastic modulus of the bonding layer on an electronic characteristic of one of the microchips is preferably suppressed.
In the present invention, the bonding layer is a silicone-type adhesive film, which has a glass transition temperature higher than +120xc2x0 C. or lower than xe2x88x9240xc2x0 C. Therefore, the influence of the variation in the elastic modulus of the bonding layer due to the glass transition on the electronic characteristic is substantially eliminated in the temperature range between xe2x88x9240xc2x0 C. and +120xc2x0 C. Moreover, the elastic modulus in the temperature range between xe2x88x9240xc2x0 C. and +120xc2x0 C. is lowered. Therefore, the influence of a thermal stress in the one of the microchips on the electronic characteristic is suppressed.